The present invention relates to optical temperature measurement using a time rate of decay measurement.
In passive optical temperature sensing, a time-rate-of-decay (TRD) temperature sensing probe is thermally coupled to an object to measure its temperature. The probe tip includes luminescent material emitting radiation at a wavelength characteristic to the material upon excitation by radiation at another material characteristic wavelength. The luminescent material is typically in the form of a sectioned, polished crystal or a powder embedded in a binder. Both the emission and the excitation wavelengths are determined by the type of luminescent material in the probe, and the intensity of the excitation is significantly more intense than that of the emitted radiation. The luminescent emission decays substantially exponentially with time and the exponential time constant of the emission curve is responsive to the temperature.
Luminescence is characterized by light emission from matter and describes several processes resulting in such emission. The luminescent emission intensity, lifetime and frequency spectrum can be temperature dependent, and therefore one or more of these parameters may be used as a thermoresponse parameter. Fluorescence and phosphorescence are two emissive processes defined quantum mechanically. Quantum mechanics teach that an electron surrounding an atomic nucleus has specific quantized, allowed states of energy characterized by quantum numbers. One of the quantum numbers is the spin quantum number, S, which is a measure of the angular momentum of the electron orbit in the energy state. Spin orbit quantum numbers are given by S=1/2.+-.n, where n is an integer. When a magnetic field is present within the atom, a spin orbit state may split into two allowed states by a process called spin-orbit splitting. The energy difference between the split states is proportional to the atomic number. Consequently, materials with a large atomic number which also have atomic magnetic fields have multiple states between which electrons may transition.
An atom phosphoresces when an electron makes a transition from one state having a spin orbit quantum number S to a second state with the same numerical quantum number, S. Phosphorescence is characterized by a relatively long emissive duration, between a microsecond and 1.times.10.sup.3 seconds. Fluorescence occurs when the electron transition occurs between states of different numerical spin quantum number and is characterized by emissions of comparatively short lifetime, from 1.times.10.sup.-2 seconds to 1.times.10.sup.-10 seconds. In most cases, activated interstitial atomic impurities, typically called "dopant" atoms in the literature, provide free electrons. Fluorescence and phosphorescence may occur in the same material if there are a substantial number of free electrons and spin orbit coupling, since the electrons are provided with multiple energy states of differing S.
The literature defining luminescent, fluorescent and phosphorescent processes is inconsistent. Sometimes processes are identified by measurement of the duration of the emission; sometimes, they are identified by a quantum mechanical analysis of the problem. In this patent application, the terms fluorescence and phosphorescence are defined as discussed above. Additionally, luminescent materials with activation sites having a high atomic number, which may luminesce via phosphorescence, fluorescence or both processes will be exclusively called luminescent materials. Readers are referred to Kittel, Introduction to Solid State Physics, Wiley, 1976 for further reading.
In a typical TRD probe, luminescent material is located in the probe tip. Upon excitation, a luminescent emission occurs which is coupled to a detector by an optical fiber. The detector converts the emission into a current having an amplitude varying with the emissive intensity. Electronics process the detector output by various means to determine the substantially exponential time constant. The decay characteristics of the detector output is substantially exponential but may be characterized linearly or having other functionality over various intervals of time. Once the time constant is measured, a look-up table or an equation curve-fitted to empirical data is used to calculate temperature.
The form of the luminescent material in the probe has some limitations, however. Some probes have a solid piece of luminescent material in the probe tip which may be crystalline or amorphous. In this application the piece of luminescent material will be called a crystal. Such luminescent probe tip materials are called crystals in this application. Other probes have a solid piece of luminescent material made of powdered crystalline or amorphous material embedded in a binder. Still other probes have a powder without a binder in the probe tip. Sectioning and polishing can break crystals during manufacture. Furthermore, there is a wide variation in luminescent signal intensities so that the electronics are individually adjusted for each crystal probe tip. Such adjustment is labor intensive and time consuming in a manufacturing environment. While the powdered form embedded in a binder improves the amount of intensity variation between crystals compared to that of a polished crystal, the resulting emissive intensity is still orders of magnitude lower than the excitation radiation. Therefore, the embedded powdered form results in a more easily interchangeable probe tip than does the polished crystal, but is still not optimized for a manufacturing process.
Sensitivity and intensity of luminescent emission over the temperature range of interest can be optimized by proper material choice. However, the emission intensity or low sensitivity of some materials is still often comparable to noise, requiring excessive electronic amplification and subsequent distortion.
Various types of electronics are used to measure a quantity related to the time rate of decay of the emission. One method measures the time for a preselected emissive intensity to halve. This method is most successful when the signal intensity is large compared to the background noise. Digital techniques digitize the emission at a high sampling rate and curve fit the resulting sampled emission curve. One analog technique measures a phase difference between the excitation radiation and the emission and correlates the phase difference to the temperature. Some methods of measuring time rate of decay are susceptible to background level variations or drift, as well as variations in signal level.
Therefore, there is a need for a temperature probe having sufficiently strong emission intensities to preclude excessive amplification, and signal processing which is insensitive to background level and signal level variations.